1. Field of the Invention
This invention relates to a method of reading out image information stored on a recording medium, in which the image information is optically scanned in two directions, the resulting emitted, transmitted or reflected light is detected and converted to an electric output signal, the output signal is then integrated for a predetermined time and subjected to a sampling process, and a picture element of the image information is read out as a sampled-value train, and an apparatus for carrying out the method. This invention is particularly related to a method of and apparatus for reading out image information, which can effectively convert a detected light even of a low intensity to an image signal.
2. Description of the Prior Art
Techniques for detecting a weak light and reading image information are required for example in the fields of medical image processing, radiation image processing, nuclear electronics, facsimile, printing plate making by use of a laser beam, or the like.
In such fields, signals are generally passed through a low-pass filter or an integration circuit before processing in order to convert the weak light to an electric signal at as high efficiency as possible.
Further, in the field of radiation image processing, it has recently been proposed to use a stimulable phosphor as an intermediate medium for radiation image recording for the purpose of improving the efficiency and accuracy of medical diagnosis. In such a system, it is particularly important that a weak light be effectively detected and converted to an electric signal with a high signal-to-noise ratio (S/N ratio) so as to obtain a reproduced radiation image of a high quality.
The above-mentioned radiation image read-out system using a stimulable phosphor as an intermediate recording medium is disclosed for example in Japanese Unexamined Patent Publication No. 11395/1981 and U.S. Pat. Nos. 4,258,264 and 4,284,889. In the system of this type, the stimulable phosphor is exposed through a human body to a radiation to have a radiation image stored thereon, the stimulable phosphor is then scanned at a high speed with a stimulating ray to cause it to emit light therefrom in the pattern of the stored image, and the light emitted from the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric signal. The obtained electric signal is processed as desired to reproduce a visible image having an image quality suitable for viewing and diagnosis purposes.
In the system described above, light emitted from the stimulable phosphor upon stimulation thereof according to the stored radiation energy amount is of a very low intensity. Therefore, the electric signal obtained by the photoelectric conversion of the detected light is greatly affected by random noise (i.e. quantum noise) of the emitted light.
Accordingly, in the system described above, an optical system which can effectively guide the detected light to the photoelectric conversion system is adopted and, in addition, a low-pass filter and an integration circuit are used to improve the use efficiency of the photoelectrically converted information.
However, the low-pass filter works on the basis of the time constant of a capacitor-resistor and, consequently, presents a very real problem with regard to a signal sampling process. Namely, signal samples cannot be completely separated from each other, so that a preceding sample adversely affects the subsequent sampling output signal. Further, the use efficiency of light information is low because of low integration efficiency.
As for the integration circuit, it is essential that the integrator be reset to prepare for the next sampling. Accordingly, the electric signal corresponding to light information which is input during the resetting period cannot be integrated. More specifically, in order to sequentially integrate the input signal for a predetermined period and determine and sample the mean value or the total value of the integrated signal, it is necessary that the integrator be reset to the initial state each time the predetermined integration period runs out. However, it takes 1 to 2.mu.s for even the most rapid switch presently available to be reset. A resetting period of this order greatly interferes with desired high-speed sampling. Thus the integration efficiency in the sampling region is low and it is impossible to improve the S/N ratio.
On the other hand, in order to maximize the integration efficiency, one might consider prolonging the integration period as much as possible within the sampling region and conducting the sampling at the end of integration, i.e. when the integrator is reset. However, if sampling is carried out to match the reset timing of the integrator, spike-like noise occurs due to the floating capacity of the reset switch and this noise adversely affects the sampling output signal. Thus, this method requires complicated and expensive circuitry and troublesome adjustment for eliminating the adverse effect of spike-like noise, and is not suitable for practical use.